In recent years, thanks to the development of digital technology of digitizing various information signals such as video and audio signals, a digital broadcasting system has been put into practical use, as well as the analog TV broadcasting. In such a digital broadcasting system, the MPEG2 system has been employed. In the MPEG2 system, contents data such as digitized video and audio data are packetized into transport stream packets (also called as “TS packets”). The TS packets are multiplexed, and transmitted as transport streams for digital broadcasting. There have been developed TV sets that enable to receive the digital TV broadcasting, as well as the analog TV broadcasting, and receivers dedicatedly used for the digital TV broadcasting called as set top boxes for allowing viewers to watch the digital broadcasting.
In the digital broadcasting system, study has been progressed regarding the terrestrial digital broadcasting. Particularly, in recent years, there is a strong demand in the market for development of movable objects as exemplified by mobile phones. In view of these circumstances, there are being developed mobile terminal devices for use in the terrestrial digital broadcasting.
There are known memory cards such as smart media and secure digital (SD) cards (hereinafter, these devices are sometimes called as “electronic cards”) in the production field of movable objects. These memory cards are detachably attachable to mobile phones, digital cameras, or the like. Further, there have been proposed special memory cards constructed such that a radio communication function or a position determining function such as a global positioning system (GPS) is equipped in the ordinary memory card (see Japanese Unexamined Patent Publication No. 2003-234935). Hereinafter, the special memory cards of this type are called as “special memory cards”. A component for realizing a special function is incorporated in such a special memory card.
FIG. 17 is an illustration showing an example of a mobile phone as a conventional mobile terminal device. The mobile phone is constructed such that a memory card or a special memory card is detachably attachable. Referring to FIG. 17, the conventional mobile terminal device includes a phone body 90, a key entering portion 92 with which a user is allowed to perform input operations such as entering of telephone numbers, and designation of various operations, a display portion 91 through which the user is notified of information regarding communication or the like, an antenna 93 for transmitting/receiving data, and various processing circuits (not shown) provided in the phone body 90. The mobile phone has an attachment portion 94 in the upper part of the phone body 90 for detachably attaching a memory card. With this arrangement, when the user inserts the memory card into the attachment portion 94, the user is allowed to use the phone body 90 in a manner of combining the mobile phone with the memory card.
For instance, when the user inserts a memory card storing image data into the attachment portion 94, and enters a command indicative of image display through the key entering portion 92, an image corresponding to the designated image data stored in the memory card is displayable on the display portion 91. Further, when the user inserts a special memory card 95 into the attachment portion 94, the user is allowed to use the phone body 90 in a manner of combining the mobile phone with the function of the special memory card. For instance, when the user inserts the special memory card 95 loaded with a GPS function into the attachment portion 94, the location of the mobile phone determined by the GPS is displayable on the display portion 91, and the user is allowed to inform the intended party of the location of the user through the function of the mobile phone. Thus, there has been proposed a system configured such that the function of the host device and the function of the special memory card are combined with each other, with the host device having the function inherent to the mobile phone, and the attachment portion 94 serving as an interface to the special memory card 95.
As the terrestrial digital broadcasting has been put into practical use, it becomes easy to receive the digital broadcasting by way of movable objects. In view of this, there has been a demand in the market for a digital broadcasting receiver or a digital broadcasting receiving system in which such a movable object is usable. Specifically, there is a demand for mobile phones or personal digital assistants (PDAs) through which the digital broadcasting is receivable, car-mounted navigation systems through which the digital broadcasting is receivable, and furthermore for a digital broadcasting receiving system in which a terrestrial digital broadcasting receiver is connected with the mobile phone or the navigation system. In view of these, a need exists in the market for a digital broadcasting receiving system in which a special memory card having a digital broadcasting receiving function, and a mobile terminal device such as a mobile phone functioning as a host device are interconnected with each other, so that users can receive and watch the digital broadcasting.
The following drawbacks should, however, be considered in establishing the digital broadcasting receiving system in which the special memory card having the digital broadcasting receiving function, and the host device are interconnected with each other.
First, since the device such as the movable object is produced on the premise of its portability, it is essential to satisfy the requirements regarding downsizing, light-weighing, and lowering of power consumption.
In addition to the above, there is a greater problem to overcome: in what way clocks in the special memory card and the host device are synchronized with each other. Particularly, there is a strong demand for developing the technology of recovering synchronization of clocks in the special memory card and the host device in a situation that the radio wave receiving condition is degraded such as the startup time of the power of the host device (or the receiver), or that the host device is located in a valley between tall buildings, as well as a situation that the user switches over the channel, or a like situation.
The latter problem will be solved if data including clock information can be transmitted from the special memory card to the host device without delay. However, in most of the cases, the interface of the special memory card as represented by an SD card adopts asynchronous communication. Therefore, the timing at which data is transmitted from the special memory card to the host device (or data is received by the host device) is not constant, namely, varies. In other words, a data transfer time is required until the host device receives data from the special memory card. Consequently, in the case where the user switches over the channel, for example, there is generated delay in time because the clock information sent from the special memory card is received by the host device with a time lag corresponding to the data transfer time. In addition to the above drawback, since the data transfer time is not constant, it is difficult to synchronize the clocks in the special memory card and the host device with each other.